1. Field of the Invention
The invention relates to the field of virtual local area networks (VLANs). In particular, the invention relates to a virtual local area network spanning multiple loop free network topology domains.
2. Background Information and Description of Related Art
A virtual local area network (VLAN) restricts communication between computers in a network on some network administrator defined basis rather than, for example, the physical location of the computers. The computers in the VLAN communicate with each other as if they are connected to the same local area network (LAN) segment even though they may actually be physically located on different segments of an internetwork. Conversely, computers connected to the same network segment may not necessarily be able to communicate with each other if, for example, they belong to different VLANs.
Networks, such as LANs, are often partitioned into multiple electrically separate segments to increase network performance. Bridges or switches connect the segments. A network may include redundant bridges and paths to provide backup routes in the event of a bridge or path failure in the network. The Spanning Tree Protocol (STP), part of the IEEE 802.1D standard, defines an algorithm that provides for this redundancy while preventing undesirable traffic loops in a LAN that can otherwise cause unwanted duplication of data packets. Using the algorithm, bridges select a root bridge, calculate path costs, and select a single path to the root bridge from among multiple redundant paths. After path costs are calculated and a single path is selected, the bridges block the redundant paths from carrying traffic unless the selected path or a bridge along the selected path fails.
There are two common approaches to running STP with VLANs. One approach, specified in the IEEE 802.1Q standard, is to run a single STP instance for all VLANs. This approach is scalable for different sizes of networks, but does not provide load-balancing capability. Since all VLANs participate in the same spanning tree, redundant paths are unused, while some active links may be overloaded. Therefore, the bandwidth capability of the network is not fully utilized.
Another approach is to run a distinct STP instance for each VLAN. This approach provides load-balancing capability, but does not scale well as the network grows. Running a separate STP instance for each VLAN may create undesirable amounts of processing and network traffic overhead and administrative burden to the network administrator.